The invention disclosed herein relates generally to variable capacitance apparatus, and more particularly to such apparatus embodied in an environmentally sealed circuit board for use in capacitively-coupled keyboards.
Capacitive keyboards are well known in the field of data input devices for information handling and data processing systems. They provide certain inherent advantages over electrical contact keyboards. These advantages particularly include mechanical simplicity, long life and absence of electrical signal problems caused by contact bounce and corrosion.
Such keyboards have been particularly popular in connection with smaller and less expensive information handling and data processing systems which cannot support the expense of keyboards based on Hall effect operation or other high technology approaches. However, there is an increasing demand in systems in which conventional capacitive keyboards are suitable for keyboard input devices which offer lower cost and higher performance. Furthermore, even in connection with high performance and high reliability data processing systems, there is increasing pressure for simpler and less expensive keyboard input devices.
Although capacitive keyboards have certain inherent advantages, even the most advanced existing capacitive keyboard designs have not been found totally suitable for high performance applications. One common performance shortcoming is susceptibility to malfunctions due to dust, moisture and other contaminants. Capacitive keyboards typically utilize capacitors having small capacitance values and require the detection of small changes in capacitance. In conventional devices variable capacitance is achieved with a movable conductive plate which is not totally sealed from the environment. Even minute amounts of solid contaminants between the movable plate and stationary portions of the device can prevent a full change in capacitance. In addition, foreign materials generally have a different dielectric value than the substance (normally air) intended to occupy the space between the plates. Accordingly, any contaminants between the plates have an additional effect on capacitance value. Finally, particularly in connection with capacitors formed of thin conductive layers on circuit boards and associated parts, liquid or gaseous contaminants in certain environments may adversely affect the layers and degrade the capacitors.
It is common to attempt to avoid some of these potential problems by sealing the areas over conductors and capacitor plates on printed circuit boards with coatings or layers of various types and configurations. Examples of this approach are shown in U.S. Pat. No. 3,797,630 issued to S. Zilkha on Mar. 19, 1974, U.S. Pat. No. 3,921,166 issued to J. Volpe on Nov. 18, 1975, U.S. Pat. No. 3,921,167 issued to J. Fox on Nov. 18, 1975, U.S. Pat. No. 4,163,222 issued to D. Gove on July 31, 1979 and British Pat. No. 2,027,209 published Feb. 13, 1980. However, the capacitive switch assemblies shown in each of these patents, except for British Pat. No. 2,027,209 as well as similar assemblies shown in U.S. Pat. No. 3,660,838 issued to D. Gove, et al, on May 2, 1972 and U.S. Pat. No. 3,951,250 issued to P. Pointon, et al, on Apr. 20, 1976, utilize an electrically conductive bridging element mounted on a movable plunger to vary the capacitive coupling between two fixed plates on a circuit board. Proper operation of such assemblies depends on repeatable predetermined minimum spacing between the bridging element and fixed plates when the plunger is depressed. In none of these examples is the bridging element located in an environmentally sealed area, nor would it be convenient to do so. Hence, contaminants may find their way into this area. As previously indicated, solid contaminants may prevent minimum spacing from being achieved. Contaminants of any type may affect the dielectric constant and hence the capacitance of individual capacitors.
The necessity for using a conductive bridging element in conventional capacitive circuit board apparatus arises because of the prior art circuit board design in which all of the circuit board mounted elements are fixed relative to one another. As shown in previously identified U.S. Pat. No. 3,921,167, it is known to use flexible dielectric substrates in capacitive circuit board apparatus. However, the design disclosed in that patent utilizes the flexibility characteristic to facilitate the fabrication of a two-sided circuit not requiring through-plated holes. The flexibility characteristic is further utilized to permit the circuit board to closely conform to a rigid support plate or substrate to produce a circuit board assembly which is flat within close tolerance as necessary to provide even contact and repeatable coupling characteristics with a movable conductive bridging element. Since the bridging element, which is not environmentally sealed, is part of the capacitance circuit, this design does not avoid certain of the previously identified problems.
Conversely, the applicants have utilized the characteristics of flexible circuit board substrates and two-sided circuit technology to achieve a unique capacitively coupled circuit board design which is especially simple and inexpensive to manufacture, and in which all of the capacitive elements are sealed from the environment surrounding the keyboard. Accordingly, many of the shortcomings of prior capacitively coupled keyboard apparatus are avoided.